1. Field of the Invention
The present invention is in the field of ceramic compositions based on oxides of sodium and tantalum and having coefficients of thermal expansion suitable for incorporating the ceramic composition into a magnetic thin film recording and reproducing head.
2. Description of the Prior Art
In the field of magnetic recording, there is a tendency towards higher density and higher frequency in the recording signals. In view of the tendency towards higher density recording, metal powder tapes have been used which employ powders of ferromagnetic metal such as Fe, Co and Ni. Also used are the evaporated type tapes which have ferromagnetic metal materials deposited by means of evaporation on a base film. These types of magnetic recording media are used with magnetic heads which must have a high saturation flux density Bs.
In using the high density recording mentioned above, the track width of the magnetic recording on the recording medium must also be reduced. This is coupled with a requirement that there is an extremely narrow track width provided on the magnetic head.
To meet these requirements, it has previously been suggested to use a composite type magnetic head employing a nonmagnetic substrate and a magnetic metal thin film which is used as the magnetic core, the nonmagnetic substrate and the thin film being stacked one on the other and the magnetic thin film is used as a track. As an alternative, a thin film type magnetic head is employed wherein the magnetic metal thin film and the conductive metal film are stacked on a nonmagnetic base using an intermediate insulating thin film. Thus, the use of metal thin films is increasing rapidly in the field of modern magnetic recording.
When a metal thin film is used in a magnetic head, a ceramic base is usually employed as a substrate. The conventional ceramic substrate has the drawback that it has a thermal expansion coefficient which is markedly lower than that of the metal film and hence the metal film tends to be detached from the ceramic base during exposure to higher temperatures. For example, the coefficient of linear expansion .alpha. of "Permalloy" or "Sendust" used in thin film magnetic heads amounts to about 130.degree. to 160.times.10.sup.-7 /.degree.C., whereas that of the commonly used barium titanate and calcium titanate ceramic bases is usually considerably lower, amounting to about 90.degree. to 100.times.10.sup.-7 /.degree.C. and 100.degree. to 120.times.10.sup.-7 /.degree.C., respectively.
When a metal thin film having some magnetostriction is used in the thin film type magnetic head, a strain is introduced into the metal film during the manufacturing process, such as physical vapor deposition or cooling after heat treatment. This metal strain causes a magnetic anisotropy to be introduced into the metal thin film because of the magnetostriction, so that its permeability is deteriorated. Thus, there is a need for a ceramic base having a linear expansion coefficient which is comparable to that of the metal thin film itself.